Process for calibrating a scanner

ABSTRACT

In a process for calibrating an optoelectronic scanning member of a scanner which scans an image point by point and line by line and converts the modulated scanning light into image values in a light/voltage converter, converter densities are first determined and stored as a converter density table depending on different degrees of amplification of the light/voltage converter. The densities of the scanning diaphragms and grey filters used are also determined and stored as diaphragm density and grey filter density tables. A scanner-specific calibration is carried out with a reference diaphragm by adjusting the degree of amplification so that the image value corresponds to a predetermined white level. When a document-specific calibration is later carried out, the required amplification is automatically determined from the stored density tables and the white point density of the document in question. The light/voltage converter is then adjusted accordingly. This calibrating process considerably shortens the time of preparation required for scanning a document and discharges the operator from routine calibration tasks.

BACKGROUND OF THE INVENTION

The invention is in the field of electronic reproduction technology andis directed to a method for calibration of an optoelectronic scannerelement of a scanner device for point-by-point and line-by-line scanningof image originals. The a scanner device, also referred to as scanner,can be a black-and-white scanner for scanning black-and-white imageoriginals, or can be a color scanner for scanning chromatic imageoriginals.

Given a black-and-white scanner, a black-white image original isilluminated pixel-by-pixel by a light source and the scan lightmodulated by the brightnesses of the scanned pixels is converted with anoptoelectronic converter into an image signal that represents thebrightness values of the scanned image original between “black” and“white”.

Given a color scanner, the scan light coming from the image original isfirst resolved with dichroitic filters into red, green and blue partsand is supplied to the three color channels of a color scanner. Thechromatic light parts are then converted with optoelectronic transducersinto three color signals that represent the color parts “red”, “green”and “blue” of the pixels scanned in the color original.

The image signals or, color signals are further-processed in signalediting stages. The signal editing stages have a defined signal inputrange of which one corner value is referred to as a white level.

The total range of an image original to be scanned is matched to thedefined signal input range of the signal editing stages by a calibrationof a black-and-white scanner or color scanner before the beginning ofscanning, in that the scan light coming from the brightest location ofthe image original, the white point, is converted into an image signalvalue or, into a color signal value per color channel that correspondsto the white level.

DE-A-25 45 961 has already disclosed a method for the automaticcalibration of scanners. In a calibration phase, the color scannerelement of a black-and-white scanner is positioned to the respectivewhite point of the image original, and the scan light coming from thetargeted white point is converted in the optoelectronic transducer intoan actual image signal value. The actual image signal value is comparedin a control unit to a rated image signal value that corresponds to thewhite level. A control signal modifies the gain of the optoelectronictransducer and/or of a following amplifier until the repetitive error iszero. The control signal value required for this purpose is stored forthe duration of the original's scanning that follows the calibrationphase. The control unit is expanded to the three color channels for thewhite balance given color scanners.

The known method has the disadvantage that a corresponding white pointon the image original to be reproduced must always be approached withthe color scanner element in the calibration, this time-consuming and,particularly given repetitions of the a white balancing, beingimprecise. Added thereto is that a brightest image location suitable aswhite point is often not present in a chromatic image original.

EP-A-0 281 659 recites a further method for the calibration of scanners,whereby the repeated approach of a white point with the color scannerelement on an image original to be reproduced is avoided. For thatpurpose, a light attenuation factor is determined by optoelectronicscanning of the white point in the initial white balancing. Givenrepetitions of the white balancing, the scan light coming from the whitepoint is simulated by the light of the light source attenuatedcorresponding to the identified light attenuation factor without renewedscanning of the white point in the image original, whereby the lightattenuation is undertaken with a controlled iris diaphragm.

The known method is complicated and is based on a color-neutral densitysimulation, which is not always established in practice, and cantherefore occasionally lead to unsatisfactory results. Further, nodensity simulation in the scanning of opaque originals is possible giventhe known method since the iris diaphragm is required for the correctsetting of the depth of field.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve a methodfor the calibration of an optoelectronic scanner element of a scannerdevice for point-by-point and line-by-line scanning of image originalssuch that a calibration that can be implemented simply and in a shorttime is enabled.

This object is achieved according to the invention by providing a methodfor calibration of an optoelectronic scanner element of the scannerdevice for point-by-point and line-by-line scanning of the image,originals. The image original is illuminated and the scan lightmodulated with densities of the scanned image original is converted intoimage values with a light/voltage transducer unit. The white level isprescribed. The calibration of the scanner element is undertaken bychanging a gain of the light/voltage transducer unit such that the imagevalue generated when scanning the brightest location of the imageoriginal, the white point, corresponds to the predetermined white level.The light/voltage transducer unit is charged with a calibration light.The transducer densities as a criterion for attenuation of thecalibration light respectively simulated by the different gains aremeasured from the measured image values given different gains of thelight/voltage transducer unit. The identified transducer densities areallocated to the corresponding gains as a transducer density table.Diaphragm densities as a criterion for the attenuation of thecalibration light achieved with the respective scanned diaphragms aredetermined from the image values that were measured given different scandiaphragms in the calibration light. The identified diaphragm densitiesare allocated to the corresponding scan diaphragms as a diaphragmdensity table. A reference diaphragm on the corresponding referencediaphragm density are determined from the diaphragm density table.Calibration is implemented with the identified reference diaphragm bysetting the gain of the light/voltage transducer unit such that theimage value acquired with the calibration light attenuated by thereference diaphragm corresponds to the predetermined white level. Thereference transducer density belonging to the gain that has been set isdetermined from the transducer density table. The scan diaphragm forscanning the image original and the corresponding diaphragm density aredetermined from the diaphragm density table. An overall density iscalculated from the reference diaphragm density, the referencetransducer density, the diaphragm density of the scan diaphragm, andfrom the density of the white point of the image original. A gain of thelight/voltage transducer unit that is allocated in the transducerdensity table to that transducer density that corresponds to thecalculated overall density is determined. The gain that has beendetermined is set at the light/voltage transducer unit for scanning theimage original.

The calibration method of the invention is composed of device-specificand master-specific steps.

In the device-specific steps, the characteristic of the light/voltagetransducer unit is first registered, and the densities of the scandiaphragms and gray filters are identified and stored in the form ofvalue tables. Subsequently, a device-specific, automatic calibrationoccurs. The device-specific steps need only be advantageouslyimplemented at great time intervals or given a replacement of componentparts in the scanner devices.

In the master-specific steps, the required gain of the light/voltagetransducer unit is automatically identified merely from the previouslystored value tables and the respective white point density of the imagemaster to be scanned and is set at the light/voltage transducer means.

The preparation time for the originals' scanning is significantlyshortened as a result of the calibration method and the operator isrelieved of routine calibration jobs.

The invention is explained in greater detail below with reference to theexample of a black-and-white scanner on the basis of the drawing figure.

BRIEF DESCRIPTION OF THE DRAWING

The drawing FIGURE shows a schematic block circuit diagram of ablack-and-white scanner with a calibration unit. An image original (2)in the form of an opaque or transparency image original that is scannerpoint-by-point and line-by-line by an optoelectronic scanner element (3)is mounted on a scanner drum (1) composed of transparent glass that isonly shown excerpted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For point-by-point illumination of the transparency image original (23),a transparency illumination unit (4) having a light source (5) and alens (6) is arranged in the inside of the scanner drum (2). For apoint-by-point illumination of the opaque image original (2), an opaqueillumination unit (7) having, for example, two light sources (8) and twolenses (9) is located in the optoelectronic scanner element (3).

The scan light allowed to pass by the transparency image original (2) orreflected by the opaque image original (2), and which is modulatedaccording to the brightnesses of the picture elements scanned in theimage original (2), proceeds via lens 161 into the scanner element (3).Therein, the scan light is focused with a scanner objective (10) ontoone of the scanner diaphragms (11) of a rotatable diaphragm wheel (12).The diaphragm wheel (12) comprises a plurality of scanner diaphragms(11) having different diameters. One criterion for the light attenuationachieved with a scanner diaphragm (11) is the diaphragm density D_(B).

The diaphragm wheel (12) is followed by a rotatable filter wheel (13)having a plurality of gray filters (14) of different strength. Onecriterion for the light attenuation achieved with a gray filter (14) isthe gray filter density D_(G).

The diaphragm wheel (12) is positioned by a diaphragm wheel drive (15)and the filter wheel (13) is positioned by a filter wheel drive (16)such that a scanner diaphragm (11) of the diaphragm wheel (12)respectively selected according to the desired light attenuation and,potentially, a gray filter (14) of the filter wheel (13) in addition arepositioned in the beam path of the scan light. Diaphragm wheel drive(15) and filter wheel drive (16) are driven by control signals S_(B) andS_(G) on lines (18, 19).

Via, for example, a collimation optics (17); the scan line proceeds ontoa light/voltage transducer unit (20, 21, 22) that is composed of a photomultiplier (20), a following current/voltage converter (21) and ahigh-voltage generator (22) in the exemplary embodiment. The gain of thephoto multiplier (20) is set via the high-voltage U_(H) that isgenerated in the high-voltage generator (22). The high-voltage generator(22) is controlled by control signal values S_(H) on a line (23) that,thus, determine the gain of the photo multiplier (20) via thehigh-voltage U_(H).

The current/voltage converter (21) generates analog voltage values U_(B)from the output current of the photo multiplier (20), said analogvoltage values U_(B) being converted in a following A/D converter (24)into digital voltage values U*_(B).

The A/D converter (24) is followed by a switchover unit (25) having theswitch positions “calibration” and “scanning”.

During the calibration of the black-and-white scanner, the scannerelement (3) is positioned outside the image original (2) on thetransparent glass of the scanner drum (1). In this position of the scanelement (3), the unmodulated light generated by the light source (5) ofthe transparency illumination unit (4) proceeds directly as calibrationlight into the scanner element (3). The calibration light is attenuatedin the scanner element (3) by a selected scanner diaphragm (11) of thediaphragm wheel (12) and, potentially, is additionally attenuated by aselected gray filter (14) of the filter wheel (13). The digital voltagevalues U*_(B) generated by the attenuated calibration light givendifferent gains of the light/voltage transducer unit (20, 21, 22) aresupplied to a calculating stage (27) in a calibration unit (28) via theswitchover unit (25) in the switch position “calibration” and via a line(26).

The calculating stage (27) has an interactive connection to ahigh-voltage density table memory (29), with a diaphragm density tablememory (30) and with a gray filter density table memory (31). Thecalibration unit (28) also comprises a digital comparator (32) forcomparing actual voltage values to a rated voltage value, comprises acontrol signal generator (33) with a following D/A converter (34) forgenerating the control signal values S_(H) for the high-voltagegenerator (22) and also comprises a central controller (35). The centralcontroller (35) has an interactive connection via a line (36) with thecalculating stage (27) and with the control signal generator (33) via aline (37). The central controller (35) also generates the controlsignals S_(B) and S_(G) for the diaphragm wheel drive (15) and thefilter wheel drive (16) on the lines (18, 19).

During the originals' scanning, the switchover means (25) is in theswitch position “scan” in which the digital voltage values U*_(B)acquired by point-by-point and line-by-line scanning of the imageoriginal (2) are stored in an image store (38) of an image processingstage (39) as digital image values for further-processing. The imageprocessing stage (39) also comprises an originals' analysis unit (40)that communicates with the image store (38). The density D_(W) of thewhite point, the brightest location in the image master (2), isdetermined in the originals' analysis unit (40) from the image valuesU*_(B) of the image original (2) stored in the image store (38). Thewhite point density D_(W), also called calibration value, is supplied tothe calculating stage (27) in the calibration unit (28) via a line (41).

Given a color scanner, a color splitter is additionally located in thescanner element, said color splitter resolving the chromatic scan lightcoming from the scanned, chromatic color original (2) into the threecolor parts “red” (R), “green” (G) and “blue” (B) and supplying them tothree separate color channels. The three color channels each comprise alight/voltage transducer unit (20, 21, 22), an A/D converter (24) and aswitchover means (25). The three color values R, G and B that areoptionally supplied to the common image processing stage (39) or to arespective calibration unit (28) allocated to each and every colorchannel are generated as digital voltage values U*_(B) in the threecolor channels.

The inventive method for calibrating an optoelectronic scanner device isset forth in greater detail below, again with reference to the exampleof a black-and-white scanner on the basis of method steps [A] through[G]. Given a color scanner, the method is analogously applied to each ofthe three color channels.

In a method step [A], a characteristic transducer characteristic D_(H)=f(S_(H)) is determined for the light/voltage transducer unit (20, 21, 22)in that, first, the relationship between a plurality of control signalvalues S_(H) for the high-voltage setting and the voltage values U*_(B)measured at the output of the light/voltage transducer unit (20, 21, 22)is determined and then what is referred to as high-voltage densitiesD_(H) are then calculated from the measured voltage values U*_(B). In acertain sense, the high-voltage densities D_(H) are a criterion for alight attenuation simulated via the gain of the light/voltage transducerunit (20, 21, 22).

For implementation of method step [A], the scanner element (3) is firstpositioned outside the image original (2) on the transparent glass ofthe scanner drum (1), so that the light of the light source (5) proceedsinto the scanner element (3) as calibration light. Moreover, a suitablescan diaphragm (11) of the diaphragm wheel (12) is selected and pivotedinto the beam path of the calibration light by turning the diaphragmwheel (12) with the diaphragm drive (15). The scan diaphragm (11) isselected such that the calibration light coming from the light source(5) and attenuated by the scan diaphragm (11) causes no over-modulationof the light/voltage transducer unit (20, 21, 22) or, respectively, ofthe following A/D converter (24). On the other hand, the lightattenuation dare only be so strong that the measured voltage valuesU*_(B) at the output of the light/voltage transducer unit (20, 21, 22)still allow an exact measured result.

After this, the control signal generator (33) successively calls controlsignal values S_(H) that decrease in graduated fashion and that areconverted in the high-voltage generator (22) into a correspondingplurality of decreasing high-voltage values U_(H). For example, thecontrol signal values S_(H) from 4000 through 0 are called, these thenbeing converted into the high-voltage values U_(H) from 800 V through300 V of the high-voltage range. The voltage values U*_(B) measured forthe individual high-voltage values U_(H) are supplied via the line (26)to the calculating stage (27) wherein the corresponding high-voltagedensities D_(H) are then calculated for the transducer characteristicD_(H)=f (S_(H)).

In the calculation of the high-voltage densities D_(H), the high-voltagedensity D_(H) allocated to the maximum control signal value S_(Hmax) isset to zero, and the high-voltage densities D_(H) for the decreasingcontrol signal values S_(H) are then calculated as a respectivelogarithmized quotient from a currently measured voltage valueU*_(B(n+1)) and from the previously measured voltage value U*_(B(n)),being calculated according to equation [1].

 D _(H(n))=log U* _(B(n+1)) /U* _(B(n))  [1]

When the measured voltage values U*_(B) nonetheless become to small inthe determination of the high-voltage densities D_(h), the previouslyselected scan diaphragm (11) of the diaphragm wheel (12) can be enlargedduring the ongoing calculations. In this case, the first high-voltagedensity D_(H) calculated with the enlarged scan diaphragm (11) is againset to zero and one then proceeds as described. The offset that hasarisen due to the renewed resetting must be compensated in thedetermination of the ultimate transducer characteristic D_(H)=f(S_(H)).

The high-voltage densities D_(H) calculated in the calculating stage(27) are deposited in the high-voltage density table memory (29) of thecalibration unit (28) as value table D_(H)=f(S_(H)) for furtheremployment, being addressable by the corresponding control signal valuesS_(H).

An example of such a value table D_(H)=f(S_(H)) for a black-and-whitescanner or for one color channel of a color scanner is reproduced below.

Control Signal High-Voltage Values S_(H) Density D_(H) 4000 0.00 38000.11 3600 0.21 3400 0.32 3200 0.42 3000 0.53 2800 0.63 2600 0.74 24000.84 2200 0.95 2000 1.05 1800 1.16 1600 1.26 1400 1.37 1200 1.47 10001.58 800 1.68 600 1.79 400 1.89 200 2.00 0 2.10

In a method step [B], the diaphragm density D_(B) for each scandiaphragm (11) of the diaphragm wheel (12) is determined in the form ofa diaphragm density table for the light attenuation achieved with thecorresponding scan diaphragm (11).

For implementation of method step [B], the scan element (3) is againpositioned on the transparent glass of the scan drum (1), so that thelight of the light source (5) again proceeds into the scanner element(3) as calibration light. Moreover, a high-voltage value U_(H) thateffect a gain of the light/voltage transducer unit (20, 21, 22) suitablefor the acceptance of the diaphragm density table is set via a controlsignal value S_(A) generated in the control signal generator (33).

Subsequently, the individual scan diaphragms (11) of the diaphragm wheel(12) that are identified by diaphragm numbers are successively pivotedinto the beam path of the calibration light by turning the diaphragmwheel (12) with the assistance of the diaphragm wheel drive (15). Thegray filter (14) of the filter wheel (13) is selected such that noover-modulation of the light/voltage transducer unit (20, 21, 22)occurs. The calibration light attenuated in this way is converted in thelight/voltage transducer unit (20, 21, 22) into voltage values U*_(B)that are supplied via the line (26) to the calculating stage (27).

The diaphragm density table is determined in the calculating stage (27).To that end, the diaphragm density D_(B) of the scan diaphragm (11)having the largest diaphragm aperture is set to zero, and the otherdiaphragm densities D_(B) are respectively calculated as a logarithmizedquotient from the currently measured voltage value U*_(B(n+1)) and fromthe previously measured voltage value U*_(B(n)), being calculatedaccording to equation [2], and the diaphragm numbers of thecorresponding scan diaphragms (11) are allocated to the calculateddiaphragm densities D_(B.)

D _(B(n))=log U* _(B(n+1)) /U* _(B(n))  [2]

The diaphragm densities D_(B) of the scan diaphragms (11) calculated inthe calculating stage (27) are deposited for further employment in thediaphragm density table memory (30) of the calibration unit (28) as adiaphragm density table D_(B)=f (diaphragm number) addressable by thecorresponding diaphragm numbers of the scan diaphragms (11).

An example of a diaphragm density table for a black-and-white scanner orfor one color channel of a color scanner is reproduced below.

Diaphragm No. Diaphragm Density D_(B) 1 3.2041 2 3.0706 3 2.9371 42.8036 5 2.6701 6 2.5366 7 2,4031 8 2,2696 9 2.1361 10 2.0026 11 1.869113 1.6021 14 1.4686 15 1.3350 16 1.2015 17 1.0680 18 0.9345 19 0.8010 200.6675 21 0.5340 22 0.4005 23 0.2670 24 0.1335 25 0.0000

In a method step [C], the gray filter density D_(G) is determined in theform of a gray filter density table for each gray filter (14) of thefilter wheel (13) as a criterion for the light attenuation achieved withthe corresponding gray filter (14).

Method step [C] is executed fundamentally as described under method step[B]. The individual gray filters (14) of the filter wheel (13)identified by filter numbers are pivoted into the beam path of thecalibration light by turning the filter wheel (13) with the assistanceof the gray filter drive (16). The scan diaphragm (11) is selected suchthat no over-modulation of the light/voltage transducer unit (20, 21,22) occurs. The calibration light attenuated in this way is converted inthe light/voltage transducer unit (20, 21, 22) into voltage valuesU*_(B) that are likewise supplied via the line (26) to the calculatingstage (27).

The gray filter density table is determined in the calculating stage(27). For that purpose the gray filter density D_(G) of the gray filter(14) having the lowest light attenuation factor is set to zero, and theother gray filter densities D_(G) are likewise respectively calculatedas a logarithmized quotient from the currently measured voltage valueU*8_(B(n+1)) and from the previously measured voltage value U*_(B(n)),being calculated according to equation [3], and the filter numbers ofthe corresponding gray filters (14) are allocated to the calculated grayfilter densities D_(G).

D _(G(n))=log U* _(B(n+1)) /U* _(B(n))  [3]

The gray filter densities D_(GF) calculated in the calculating stage(27) are deposited for further employment in the gray filter densitytable memory (31) of the calibration unit (28) as gray filter tableD_(G)=f (filter number) addressable by the corresponding filteridentification numbers of the gray filters (14).

An example of a gray filter density table for a black-and-white scanneror for one color channel of a color scanner is reproduced below.

Filter No. Gray Filter Density D_(G) 1 0.0000 2 0.3000 3 0.6000 4 0.90005 1.2000 6 1.5000 7 1.8000 8 2.1000

After the calculation of the three density tables, a scan diaphragm (11)of the diaphragm wheel (12) to be employed as the reference diaphragmand the corresponding reference diaphragm density D_(RB) of thereference diaphragm from the diaphragm density table for thedevice-specific calibration of the light/voltage transducer unit (20,21, 22) are determined in a method step [D].

For the implementation of method step [D], a minimum high-voltage valueU_(H) that effects a gain of the light/voltage transducer unit (20, 21,22) suitable for the determination of the reference diaphragm is set viaa control signal value S_(H) generated in the control signal generator(33). Moreover, it is also seen to that no gray filter (14) of thefilter wheel (13) is positioned in the beam path of the calibrationlight. The individual scan diaphragms (11) of the diaphragm wheel (12)are then successively pivoted in with the diaphragm wheel drive (18),and the corresponding voltage values U*_(B) are measured and evaluated.

Given a black-and-white scanner, that scan diaphragm (11) having thelargest diaphragm aperture that still just supplies a voltage valueU*_(B) lying below a limit voltage value U_(G) is selected as areference diaphragm. Expediently, the predetermined white level U_(W) isselected as a limit voltage value U_(G). The reference diaphragm densityD_(RB) belonging to the selected reference diaphragm is taken from thediaphragm density table deposited in the diaphragm density table memory(30) and is correspondingly marked therein. When, for example, the scandiaphragm (11) having diaphragm number “8” is selected as the referencediaphragm, this has the reference diaphragm density D_(RB)=2.2696.

Given a color scanner, that scan diaphragm (11) that supplies a voltagevalue U*_(B) lying below the limit voltage value U_(G) in all threecolor channels is selected as the reference diaphragm.

In a method step [E], the device-specific calibration of thelight/voltage transducer unit (20, 21, 22) to a predetermined voltagelevel, preferably to the predetermined white level U_(W), givenpositioning of the scanner element (3) on the transparent glass of thescanner drum (1) is then implemented with the reference diaphragm (11)identified in method step [D] and without interposition of a gray filter(14).

In the device-specific calibration, the calibration light attenuated bythe reference diaphragm (11) that has been set proceeds onto thelight/voltage transducer unit (20, 21, 22) that converts the attenuatedcalibration light into a voltage value U*_(B) as current actual voltagevalue U_(IST) for a control. The respective actual voltage value U_(IST)proceeds via the switchover unit (25) in the switch position“calibration” and via the line (26) onto a first input of the digitalcomparator (32). The respectively current actual voltage value U_(IST)is compared in the digital comparator (32) to the predetermined whitelevel U_(W) present at a second input of the digital comparator (21) asrated voltage value U_(SOLL).

Dependent on the current comparison result achieved in the digitalcomparator (32), which is supplied via a line (42) to the control signalgenerator (33), the control signal generator (33) generates increasingor decreasing control signal values S_(H). The increasing or decreasingcontrol signal values S_(H), increase or reduce the gain of the photomultiplier (20) via the high-voltage values U_(H) and, thus, the actualvoltage value U_(IST) until this corresponds to the rated voltage valueU_(SOLL) and the control signal generator (33) is turned off.

The gain of the photo multiplier (20) required for the coincidencebetween actual voltage value U_(IST) and predetermined white level U_(W)is kept constant by storing the corresponding reference control signalvalue S_(RH) in the control signal generator (33). What thedevice-specific calibration achieves is that the predetermined whitelevel U_(W) is always achieved at the output of the light/voltagetransducermeans (20, 21, 22) when scanning on the transparent glass ofthe scanner drum (1) with the reference diaphragm (11).

At the end of the device-specific calibration, moreover, the referencehigh-voltage density D_(RH) previously calculated for the storedreference control signal value S_(RH) is also determined from thedeposited high-voltage density table and, together with thecorresponding reference control signal value S_(RH), is stored in thecalculating stage (27) for further employment. When, for example, theidentified reference control signal value S_(RH) amounts to 400, thereference high-voltage density DRH=1.89 is taken from the high-voltagedensity table for this.

The device-specific calibration according to method steps [A] through[E] need only be repeated at large time intervals or when components arereplaced, for example given replacement of a light source or of thephoto multiplier.

The subsequent setting of the gain of the light/voltage transducer unit(20, 21, 22) according to method steps [F] and [G], by contrast, isdependent on the original and must therefore be implemented given everynew image original (2) to be scanned.

In method step [F], the required scan diaphragm (11) for scanning theimage original (2) and the required control signal value S_(H) for thehigh-voltage generator (22) are determined for a second,master-dependent calibration of the light/voltage transducer unit (20,21, 22).

The selection of the required scan diaphragm (11) of the diaphragm wheel(12) occurs according to the desired scan resolution in a fine scanningor a rough scanning of the image original (2). At the same time, thediaphragm density D_(B) of the selected scan diaphragm (11) is takenfrom the stored diaphragm density table.

Among other things, the density D_(W) of the white point of the imageoriginal (2) to be respectively scanned is required for determining therequired control signal value S_(H) of the high-voltage generator (22)of the light/voltage transducer unit (20, 21, 22). The white pointdensity D_(W) can be previously determined with a separate jobpreparation device or with the scanner device.

The white point density D_(W) can be determined by manual, densimetricmeasuring of the white point of the image original (2) with the scannerelement (3) or on the basis of an automatic analysis of the image scopeof the image original (2) on the basis of the image values U*_(B)acquired by point-by-point and line-by-line scanning of the imageoriginal (2).

For automatic analysis of the image scope, the scanner element (3) firstscans the image original (2) point-by-point and line-by-line. The imagevalue is thereby acquired are stored in the image store (38) of theimage processing means (39) via the switchover means (25) in the switchposition “scan”. The white point density D_(W) of the image original (2)is determined in the originals' analysis unit (40) on the basis of theimage values U*_(B) deposited in the image store (38) and is forwardedto the calculating stage (27) in the calibration unit (28) via the line(41).

The originals' analysis can occur, for example, according to DE-A-43 09879. The image values U*_(B) for the originals' analysis can be acquiredby a fine scanning (fine scan) or by a rough scanning (pre-scan or roughscan) of the image original (2). Given fine scanning, the image original(2) is scanned with the scan resolution required for the reproduction;in the rough scanning, it is scanned with a correspondingly coarser scanresolution and with a scan diaphragm that is enlarged compared to thenormal scan diaphragm.

The calculation of the control signal value S_(H) to be set ensues asfollows after the determination of the white point density D_(W) of theimage original (2).

First, the diaphragm density difference ΔD_(B) between the referencediaphragm density D_(RB) of the reference diaphragm identified in methodstep [D] and the diaphragm density D_(B) of the scan diaphragm selectedfor the rough scanning or fine scanning is determined according toequation [4].

ΔD _(B)=(D _(RB) −D _(B))  [4]

An overall density D_(GS) is calculated according to equation [5] fromthe identified diaphragm density difference ΔD_(B), the white pointdensity D_(W), the reference high voltage density D_(RH) determined inthe first calibration in method step [E] and an offset density D_(O)that takes potential fluctuations of the light intensity of the lightsource (5) into consideration.

D _(GS) =ΔD _(B) +D _(RH) −D _(W) −D _(O)  [5]

Denoting in equation [5] are:

D_(GS)=overall density

ΔD^(B)=diaphragm density difference

D_(RH)=reference high-voltage density

D_(W)=white point density

D_(O)=offset density.

The offset density D_(O), which is a criterion for the deviation of thelight intensity of the light source (5) from a rated value, isdetermined, for example, via a control measurement on transparent glassand need be updated only at greater time intervals, for example whenreplacing the light source or the scanner drum.

When the master-dependent gain setting is to be implemented for a finescanning of the image original (2), the white point density D_(W)determined in the analysis of the original is inserted into equation[5]. When the master-dependent gain setting is to be implemented for arough scanning for the purpose of an analysis of the original, the whitepoint density D_(W)=0 is inserted into equation [5] since the whitepoint density D_(W) of the image original (2) is still unknown.

After the calculation of the overall density D_(GS) as calibrationdensity for the master-related gain setting, the control signal valueS_(H) that is allocated to that high-voltage density D_(H) thatcorresponds in terms of value to the calculated overall density D_(GS)is marked in the deposited high-voltage density table D_(H)=f(S_(H)).The marked control signal value S_(H) is then employed for setting thegain of the a light/voltage transducer unit (20, 21, 22).

When the calculated overall density D_(GS) happens to lie outside thevalue range of the high-voltage densities D_(H) of the high-voltagedensity table, a gray filter (14) of the filter wheel (13) is pivoted inin order to reduce the overall density D_(GS) such that it lies withinthe value range of the high-voltage densities D_(H). The requiredminimum gray filter density D_(Gmin) is calculated according to equation[6], being calculated from the diaphragm density difference ΔD_(B), thewhite point density D_(W) of the image original (2) and from the maximumhigh-voltage density D_(Hmax) in the high-voltage density table.

D _(Gmin) =D _(GS) −D _(Hmax)  [6]

The gray filter density D_(G) of a gray filter (14) coming closest tothe calculated gray filter density D_(Gmin) is taken from the grayfilter density table and the corresponding filter number of the grayfilter (14) to be employed is identified.

The reduced overall density G*_(DS) then derives according to equation[7].

D* _(GS) =D _(GS) −D _(G)  [7]

Denoting in equation [7] are:

D*_(GS)=reduced overall density

D_(GS)=calculated overall density

D_(G)=gray filter density of the employed gray filter.

When the desired values cannot be directly taken from the value tables,they can be calculated from the values present in the value tables onthe basis of a simple linear interpolation calculation.

After the determination of the setting parameters, the master-relatedsetting of the gain of the light/voltage transducer unit (20, 21, 22)occurs in the final method step [G], to which end, dependent on whethera fine scanning or a rough scanning of the image original (2) is tooccur, the required scan diaphragm (11) and, potentially, the requiredgray filter (14) must be brought into the beam path of the scan lightand the correspondingly calculated setting parameters must be set.

In conclusion, the invention shall be illustrated with reference to twoexamples.

EXAMPLE 1

When the scan diaphragm having the diaphragm number “5” is selected, thecorresponding diaphragm density D_(B)=2.6701 derives from the diaphragmdensity table.

When the scan diaphragm having the diaphragm number “8” was determinedas the reference diaphragm, the reference diaphragm densityD_(RB)=2.2696 derives from the diaphragm density table.

The diaphragm density differenceΔD_(B)=(D_(RB)−D_(B))=2.2696−2.6701=−0.4005 is obtained from equation[4].

When it is also assumed that a reference control signal value S_(RH)=400is found in the device-specific calibration and the referencehigh-voltage density D_(RH)=1.89 was taken therefor in the high-voltagedensity table and that a white point density D_(W)=0.2000 and an offsetdensity D_(O)=0.0700 were found, the overall density results asD_(GS)=ΔD_(B)+D_(RH)−D_(W)−D_(O)=−0.4005+1.8900−0.2000−0.0700=1.2195according to equation [5]. The calculated overall density D_(GS) lieswithin the value table, and no additional light attenuation is required,so that the gray filter (14) having the filter number “1 ” and the grayfilter density D_(G)−0.0000 is selected.

The required control signal value S_(H)=1681 is then determined for thecalculated overall density D_(GS) by interpolation from the high-voltagedensity table.

EXAMPLE 2

When the scan diaphragm with the diaphragm number “15” is selected, thecorresponding diaphragm density D_(B)=1.3350 derives from the diaphragmdensity table.

When the scan diaphragm having the diaphragm number “8” was againdetermined as the reference diaphragm, the referenced diaphragm densityD_(RB)=2.2696 derives from the diaphragm density table.

The diaphragm density differenceΔD_(B)=(D_(RB)−D_(B))=2.2696−1.3350=0.9346 is obtained from equation[4].

When it is again assumed that a reference control signal valueS_(RH)=400 was identified in the device-specific calibration and thereference high-voltage density D_(RH)=1.89 was taken therefor in thehigh-voltage density table and that a white point density D_(W)=0.2000and an offset density D_(O)=0.0700 were identified, the overall densityderives asD_(GS)=ΔD_(B)+D_(RH)−D_(W)−D_(O)=0.9346+1.8900−0.0200−0.0700=2.5546according to equation [5].

This time, the calculated overall density D_(GS)=2.5546 lies outside thevalue range of the high-voltage density table, so that a gray filtermust be employed for reducing the density. In this case, a minimum grayfilter density D_(Gmin)=D_(GS)−D_(max)=2.5546−2.1000=0.4546 iscalculated according to equation [6] with D_(Hmax)=2.1000. The grayfilter having the filter number “3” and the gray filter densityD_(G)=0.6000 is then selected from the gray filter density table.

According to equation [7], the reduced overall density then derives asD*_(GS)=D_(GS)−D_(G)=2.5546−0.6000=1.9546 and the corresponding controlsignal value is approximately S_(H)=283.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that our wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within our contribution tothe art.

We claim:
 1. A method for calibration of an optoelectronic scannerelement of a scanner device for point-by-point and line-by-line scanningof an image original, comprising the steps of: illuminating an imageoriginal and converting scan light modulated with densities of thescanned image original into image values with a light/voltage transducerunit; predetermining a white level; undertaking the calibration of thescanner element by changing a gain of the light/voltage transducer unitsuch that an image value generated when scanning a brightest location ofthe image original, a white point, corresponds to the predeterminedwhite level; charging the light/voltage transducer unit with acalibration light; measuring transducer densities as a criterion forattenuation of the calibration light respectively simulated by differentgains from measured image values U*_(B) given different gains of thelight/voltage transducer unit, and allocating identified transducerdensities D_(H) to the corresponding gains as a transducer densitytable; determining diaphragm densities D_(B) as a criterion for theattenuation of the calibration light achieved with respective scandiaphragms from the image values U*_(B) that were measured givendifferent scan diaphragms in the calibration light, and allocating theidentified diaphragm densities D_(B) to the corresponding scandiaphragms as a diaphragm density table; determining a referencediaphragm and a corresponding reference diaphragm density D_(RB) fromthe diaphragm density table; implementing a calibration with theidentified reference diaphragm by setting the gain of the light/voltagetransducer unit such that the image value U*_(B) acquired with thecalibration light attenuated by the reference diaphragm corresponds tothe predetermined white level U_(W), and determining a referencetransducer density D_(RH) belonging to the gain that has been set fromthe transducer density table; determining the scan diaphragm forscanning the image original and the corresponding diaphragm densityD_(B) from the diaphragm density table; calculating an overall densityD_(GS) from the reference diaphragm density D_(RB), the referencetransducer density D_(RH), the diaphragm density D_(B) of the scandiaphragm and from the density D_(W) of the white point of the imageoriginal; determining a gain of the light/voltage transducer unit thatis allocated in the transducer density table to that transducer densityD_(H) that corresponds to the calculated overall density D_(GS) and;setting the gain that has been determined at the light/voltagetransducer unit for scanning the image original.
 2. The method accordingto claim 1, wherein for calculating the overall density D_(GS): thediaphragm density difference ΔD_(B) between the identified referencediaphragm density D_(RB) and the diaphragm density D_(B) of the scandiaphragm is calculated according to the equationΔD_(B)=(ΔD_(RB)−ΔD_(B)); and the overall density D_(GS) is calculatedfrom the diaphragm density difference ΔD_(B), the reference diaphragmdensity D_(RB) and the density D_(W) of the white point of the imageoriginal according to the equation D_(GS)=ΔD_(B)+D_(RH)−D_(W).
 3. Themethod according to claim 1 wherein gray filter densities D_(G) aredetermined from the image values U*_(B) that are measured givendifferent gray filters in the calibration light, being measured as acriterion for the attenuation of the calibration light achieved with therespective gray filters, and the identified gray filter densities D_(G)are allocated to the corresponding gray filters as gray filter densitytable; the calculated overall density D_(GS) is reduced by a gray filterdensity D_(Gmin) when it lies outside a value range of the transducerdensity table such that a reduced overall density D*_(GS) lies withinthe value range of the transducer density table; a required gray filterdensity D_(Gmin) is calculated by forming a difference between theoverall density D_(GS) and a maximum density value D_(Hmax) of thetransducer density table according to the equationD_(Gmin)=D_(GS)−D_(Hmax); and the gray filter allocated to thecalculated gray filter density D_(Gmin) in the gray filter density tableis identified and employed for the attenuation of the calibration light.4. The method according to claim 1 wherein values that cannot bedirectly taken from the density tables are identified by interpolation.5. The method according to claim 1, wherein for determining thetransducer density table: the transducer density D_(H) allocated to themaximum gain is set equal to zero; gains describing graduated from themaximum gain are prescribed; and the transducer densities D_(H) for thedescending gains are respectively calculated as a logarithmized quotientfrom a currently measured image value U*_(B(n+1)) and a previouslymeasured image value U*_(Bn).
 6. The method according to claim 5 whereinthe calibration light incident onto the light/voltage transducer unit inthe determination of the transducer density table is attenuated with ascan diaphragm such that no over-modulation occurs.
 7. The methodaccording to claims 1, wherein for determining the diaphragm densitytable: respective scan diaphragms having different diaphragm aperturesare brought into the calibration light; the diaphragm density D_(B) isset to zero for a scan diaphragm having a largest diaphragm aperture;and the diaphragm densities D_(B) for the various scan diaphragms arerespectively calculated as a logarithmized quotient of a currentlymeasured image value U*_(B(n+1)) and a previously measured image valueU*_(Bn).
 8. The method according to claim 7 wherein the gain is set inthe determination of the diaphragm density table such that noover-modulation occurs.
 9. The method according to claims 1 wherein fordetermining the reference diaphragm: a suitable gain of thelight/voltage transducer unit is set; the image values U*_(B) achievedgiven the various scan diaphragms are compared to a limit value U_(G);and one of the scan diaphragms is selected as a reference diaphragm onthe basis of the comparison.
 10. The method according to claim 9 whereingiven a black-and-white scanner, the scan diaphragm having the largerdiaphragm aperture is selected as a reference diaphragm with which animage value U*_(B) lying below the limit value U_(G) is achieved. 11.The method according to claim 9, wherein given a color scanner, thatscan diaphragm is selected as the reference diaphragm with which animage value U*_(B) lying below the limit value U_(G) is generated in allthree color channels.
 12. The method according to claims 9 wherein thewhite level U_(W) is selected as a limit value U_(G).
 13. The methodaccording to claim 1 wherein the scan diaphragm is selected for scanningthe image original in conformity with the desired scan resolution. 14.The method according to claim 1 wherein the optical density D_(W) of thewhite point of the image original is determined by densimetric measuringof the brightest location of the image original.
 15. The methodaccording to claim 1 wherein the image original is optoelectronicallyscanned point-by-point and line-by-line, and the image values therebyacquired are digitized and stored; and the optical density D_(W) of thewhite point of the original is determined by an analysis of the originalon the basis of the stored, digital image values.
 16. The methodaccording to claim 5 wherein the image values for the analysis of theoriginal are acquired by scanning the image original with a scanfineness that is coarser compared to the normal scan fineness.
 17. Themethod according to claim 1 wherein for determining the gray filterdensity table: respective gray filters having different lightattenuation factors are introduced into the calibration light; the grayfilter density D_(G) is set to zero for the gray filter having thehighest light attenuation factor; and the gray filter densities D_(B)for the different gray filters are respectively calculated as alogarithmized quotient of a currently measured image value U*_(B(n+1))and the previously measured image value U*_(Bn).
 18. The methodaccording to claim 17, wherein the calibration light is attenuated witha scan diaphragm in the determination of the gray filter density tablesuch that no over-modulation occurs.
 19. The method according to claim 1wherein the scan diaphragms are identified by diaphragm numbers; and theidentified diaphragm densities D_(B) of the diaphragm density table arestored addressable by the diaphragm numbers of the corresponding scandiaphragms.
 20. The method according to claims 1 wherein gray filtersare identified with filter numbers; and identified gray filter densitiesD_(G) of the gray filter density table are stored addressable by thegray filter numbers of the corresponding gray filters.
 21. The methodaccording to claim 1 wherein the light/voltage transducer unit is formedof a photo multiplier, of a following current/voltage converter and of ahigh-voltage generator controlled by a control signal value S_(H) whosehigh-voltage values determine the gains of the light/voltage transducerunit; the graduated gains are set by graduated control signal valuesS_(H); and the identified transducer densities D_(H) are stored as atransducer density table D_(H)=f(S_(H)) callable by the correspondingcontrol signal values S_(H).
 22. The method according to claim 1,wherein the light/voltage transducer unit is formed of a photomultiplier, of a following current/voltage converter and of ahigh-voltage generator controlled by a control signal values S_(H) whosehigh-voltage values determine the gains of the light/voltage transducerunit; graduated control signal values S_(H) are generated as ratedvalues for the calibration; the image values U*_(B), as actual values,are compared to the control signal values S_(H); and the control signalvalues S_(H) achieved given equality are stored.
 23. The methodaccording to claim 1 wherein the calibration light is generated by thelight source the scanner device employed for scanning the originals. 24.The method according to claim 1 wherein the method is applied to eachcolor channel of a color scanner.
 25. A method for calibration of anoptoelectronic scanner element of a scanner device for point-by-pointand line-by-line scanning of an image original, comprising the steps of:illuminating an image original and converting scan light modulated withdensities of the scanned image original into image values with alight/voltage transducer unit; predetermining a white level; undertakingthe calibration of the scanner element by changing a gain of thelight/voltage transducer unit such that an image value generated whenscanning a brightest location of an image original, a white point,corresponds to the predetermined white level; charging the light/voltagetransducer unit with a calibration light; measuring transducer densitiesas a criterion for attenuation of the calibration light respectivelysimulated by different gains from measured image values given differentgains of the light/voltage transducer unit, and allocating identifiedtransducer densities to the corresponding gains as a transducer densitytable; determining diaphragm densities as a criterion for theattenuation of the calibration light achieved with respective scandiaphragms from the image values that were measured given different scandiaphragms in the calibration light, and allocating the identifieddiaphragm densities to the corresponding scan diaphragms as a diaphragmdensity table; determining a reference diaphragm and a correspondingreference diaphragm density from the diaphragm density table;implementing a calibration with the identified reference diaphragm bysetting the gain of the light/voltage transducer unit such that theimage value acquired with the calibration light attenuated by thereference diaphragm corresponds to the predetermined white level, anddetermining a reference transducer density belonging to the gain thathas been set from the transducer density table; determining the scandiaphragm for scanning the image original and the correspondingdiaphragm density from the diaphragm density table; calculating anoverall density from the reference diaphragm density, the referencetransducer density, the diaphragm density of the scan diaphragm and fromthe density of the white point of the image original; determining a gainof the light/voltage transducer unit that is allocated in the transducerdensity table to that transducer density which corresponds to thecalculated overall density and; setting the gain that has beendetermined at the light/voltage transducer unit for scanning the imageoriginal.